YouTube's built-in Sharing and Messaging system has had a long history: after being privately tested for a many months, it went live in Canada then became official for everyone in August of 2017, and finally was bundled with notifications in the new Activity tab. However, through it all, it has remained a mobile-only feature, available to Android and iOS users. If you were on your desktop, you couldn't share a video with your YouTube contacts easily, see what they sent you, or reply to their messages. That's finally ending now.